Operational amplifiers, more commonly known as "op-amps", are the core component in analog integrated circuits ("ICs"). An ideal operational amplifier has a differential input, single-ended or differential-ended output, infinite gain, infinite input impedance and zero output impedance.
The rapidly increasing complexity of functions performed by integrated circuits has resulted in a need to implement complete analog-digital subsystems on a single integrated circuit substrate. For this reason, implementation of certain analog functions in Metal-Oxide-Semiconductor ("MOS") technology has become increasingly important. Because operational amplifiers are key elements to most analog subsystems, such as switched capacitor filters or analog-to-digital (A/D) converters, performance of such analog-digital subsystems is strongly influenced by operational amplifier characteristics.
In designing an operational amplifier, it is important to consider various advantages and drawbacks associated with each of the amplifier configurations such as common emitter ("CE"), common base ("CB") and common collector ("CC"). For instance, a specific property of an amplifier configuration can be enhanced, a certain deficiency can be remedied and/or a special feature can be added by combining two configurations together (e.g., CC-CC, CC-CE, CC-CB, CE-CE and CE-CB).
The CE-CB configuration, usually referred to as a cascode amplifier, has basically the same input characteristics as the CE amplifier and the same output characteristics as the CB amplifier. In particular, the cascode amplifier has high output resistance which can be useful in achieving a large amount of voltage gain. High frequency feedback is also limited and the Miller capacitance is reduced because the voltage gain of the CE configuration is very low. Therefore, the CE-CB (or cascode) configuration is inherently stable and well suited for high-frequency applications.
In a system comprising an analog integrated circuit or a very large mixed-mode integrated circuit consisting of analog and digital integrated circuits, one of the integrated circuits consuming a relatively large amount of power may be the operational amplifier. Accordingly, to implement a low power system, it is important to design operational amplifiers which have reduced power consumption requirements